A decoupling circuit may be connected between an IC and a power supply to prevent power-supply noise from entering the IC from the power supply. The decoupling circuit removes fluctuations in current by causing the power-supply noise to flow into the ground over a wide frequency band ranging from a low frequency to a high frequency. The decoupling circuit employs a multilayer capacitor having a low impedance characteristic between the capacitor and the ground over the wide frequency band. For example, a plurality of capacitors having different self-resonance frequencies are connected in parallel such that frequency bands where respective self-resonance frequencies of the capacitors have minimum impedances are successively arranged from the low frequency side to the high frequency side over the wide frequency band as a target.
In electronics, capacitors and inductors have parasitic inductance and capacitance, respectively. For a capacitor, the inductance is primarily due to the physical dimensions including the leads. Since a capacitor and inductor in series creates an oscillating circuit, all capacitors and inductors will oscillate when stimulated with a step impulse. The frequency of this oscillation is the self-resonant frequency (SRF). A capacitor or inductor behaves ideally only when its working frequency is well below the self-resonant frequency. As its working frequency increases, the effects of the parasitic inductance or capacitance became more pronounced until its self-resonant frequency, when the effective capacitance or inductance is zero since it is canceled by its counterpart.
Equivalent series inductance (ESL) is an effective inductance that is used to describe the inductive part of the impedance of certain electrical components. Ideally, the impedance of a capacitor falls with increasing frequency at 20 dB/decade. However, due partly to the inductive properties of the connections, and partly to non-ideal characteristics of the capacitor material, real capacitors also have inductive properties whose impedance rises with the frequency at 20 dB/decade. At the resonance frequency the sum of both is minimal, above the resonance frequency the parasitic series inductance of the capacitor dominates.
ESR is an effective resistance that is used to describe the resistive parts of the impedance of certain electrical components. ESR is properly the real resistive component of the complex impedance Z(w)=R+j X(w) of a device; this complex impedance can involve several relatively minor resistances, inductances and capacitances. These small deviations from the ideal behavior of the device can become significant when it is operating under certain conditions, i.e., high frequency, high current, or temperature extremes.
In a known multilayer capacitor, a current path length is made as short as possible to reduce Equivalent Series Inductance (ESL). However, a short current path length causes a low Equivalent Series Resistance (ESR). In the known multilayer capacitor, therefore, the impedance is extremely low near the self-resonance frequency. When this type of multilayer capacitor is used in series to form the decoupling circuit, the impedance becomes extremely high at an anti-resonance frequency formed by the capacitors having self-resonance frequencies close to each other. In this manner, the decoupling circuit cannot cause noise to flow into the ground near the anti-resonance frequency.
The above-mentioned problem with the multilayer capacitor can be overcome, for example, by electrically connecting a resistor between a lead-out portion of an internal electrode (i.e., an exposed portion of an internal electrode layer) and an external electrode for adjustment of a resistance value of the ESR. It is, however, difficult to accurately adjust the resistance value of the ESR of the multilayer capacitor after it has been manufactured. For example, it may be useful to adjust for characteristics of an IC to which the decoupling circuit is connected.
Thus, there is a need for a multilayer capacitor which has a low ESL and whose ESR can be accurately adjusted.